SECRETORY CELLS AND CAVITIES 151 



Secretions of the nature of by-products (ethereal oils, resins, 

 etc.), 1 although often deposited in glandular hairs, are frequently 

 lodged within the body of the plant. In some plants (e.g. Bay 

 Laurel, Laurus nobilis) such secretions are found in isolated cells 

 (secretory cells), often differing, apart from their contents, in shape 

 and in their larger size from the cells of the surrounding paren- 

 chymatous tissues in which they usually occur. Typical instances 

 are furnished by the so-called tannin-sacs, which are generally 

 characterised by a slightly elongated form ; extreme examples 

 are found in the cortex and pith of the Elder (Sambucus) . 



More striking are the secretory cavities, i.e. large intercellular 

 spaces, approximately isodiametric in form, and again usually 

 lodged in the parenchymatous tissues of the plant. In many 

 cases they appear as transparent dots when leaves containing 

 them are held up to the light, a phenomenon well seen in the 

 St. John's Wort (Hypericum perforatum) and in the Rue (Ruta 

 graveolens). The cavities are filled with an oily secretion, to 

 which these plants owe their peculiar odour. In cross-sections 

 of the leaves of the St. John's Wort the cavities appear more or 

 less circular, each being lined with a layer of thin-walled, some- 

 what flattened cells (the epithelium, cf. Fig. 76, B, S.), which 

 discharge the secretion into the central space. The cavities of 

 this plant originate by a gradual separation of the cells, a type 

 of development spoken of as schizogenous, and recognisable even 

 at maturity by the presence of a well-defined epithelium ; similar 

 schizogenous cavities are encountered in the leaves of the Myrtle 

 (Myrtus communis). 



In those of the Rue, on the other hand, the cavities arise by a 

 disorganisation of the secreting cells whose remains (cf . Fig. 76, A) 

 persist at the periphery, this mode of origin being described as 

 lysigenous ; thus at maturity no epithelial layer is present. 

 Similar lysigenous cavities are encountered in the heads of the 

 Clove (Eugenia caryophyllata, Fig. 76, A) and in the skin of the 

 Orange. The secretory cavities of the Rue immediately adjoin 

 the upper epidermis of the leaf, and the secretion in this instance 

 gradually escapes to the exterior through a special pore, some- 

 what resembling a stoma, except that the slit is zigzagged and 

 surrounded by four cells in place of two ; bending of the leaves 



1 Regarding the nature and function of these secretions see Chapter V . 



